1. Field of the Invention
The invention relates to a theft protection system for an automobile, in particular an immobilizer, in which control units are cleared only when a code signal is recognized as being authorized.
A theft protection system which is known from German Patent DE 43 33 474 C2, corresponding to U.S. Pat. No. 5,675,490, has one or more code generators which send their code signal to a test unit in the automobile. The test unit is connected to a plurality of control units by a data line. After the code signal has been received, the test unit requests all of the control units connected to the data line to send back their identification. The test unit checks the authorization of the identifications and, when a predetermined number of control units answer correctly, the automobile is cleared for starting.
The test unit of such a theft protection system must know, from the outset, which control units are tied into the theft protection system. Since automobiles often have different equipment, in the known theft protection system each test unit has to be prepared for the level of equipment through the use of preset software, before the test unit can carry out an authentication operation.
German Patent DE 195 26 530 C1, corresponding to U.S. Pat. No. 5,796,329, likewise discloses a theft protection system for an automobile and a method for initializing the theft protection system. In that case, a plurality of control units are tied into the system and all have to feed back their identification code word correctly before the automobile can be started. If a control unit is exchanged, the new control unit first has to be initialized. The key codes of two keys are required for this purpose. Moreover, the identifications of control units are interrogated by a test unit. Then, when both keys are accepted by the test unit and clearance is given to the test unit through a diagnostic unit, the new code words are stored in the exchanged control unit.